OPA365-EP Datasheet, PDF (11/19 Pages) Texas Instruments – 50-MHz Low-Distortion High-CMRR Rail-to-Rail I/O, Single-Supply Operational Amplifier

OPA365-EP
www.ti.com
SLOS735 – AUGUST 2011
Capacitive Loads
The OPA365 may be used in applications where driving a capacitive load is required. As with all op amps, there
may be specific instances where the OPA365 can become unstable, leading to oscillation. The particular op amp
circuit configuration, layout, gain and output loading are some of the factors to consider when establishing
whether an amplifier will be stable in operation. An op amp in the unity-gain (1 V/V) buffer configuration and
driving a capacitive load exhibits a greater tendency to be unstable than an amplifier operated at a higher noise
gain. The capacitive load, in conjunction with the op amp output resistance, creates a pole within the feedback
loop that degrades the phase margin. The degradation of the phase margin increases as the capacitive loading
increases.
When operating in the unity-gain configuration, the OPA365 remains stable with a pure capacitive load up to
approximately 1 nF. The equivalent series resistance (ESR) of some very large capacitors (C_L > 1 µF) is
sufficient to alter the phase characteristics in the feedback loop such that the amplifier remains stable. Increasing
the amplifier closed-loop gain allows the amplifier to drive increasingly larger capacitance. This increased
capability is evident when observing the overshoot response of the amplifier at higher voltage gains. See the
typical characteristic graph, Small-Signal Overshoot vs. Capacitive Load.
One technique for increasing the capacitive load drive capability of the amplifier operating in unity gain is to insert
a small resistor, typically 10 Ω to 20 Ω, in series with the output; see Figure 7. This resistor significantly reduces
the overshoot and ringing associated with large capacitive loads. A possible problem with this technique is that a
voltage divider is created with the added series resistor and any resistor connected in parallel with the capacitive
load. The voltage divider introduces a gain error at the output that reduces the output swing. The error
contributed by the voltage divider may be insignificant. For instance, with a load resistance, R_L = 10 kΩ, and
R_S = 20 Ω, the gain error is only about 0.2%. However, when R_L is decreased to 600 Ω, which the OPA365 is
able to drive, the error increases to 7.5%.
V+
R_S
OPA365
V_OUT
V_IN
10Ω to
20Ω
R_L
C_L
Figure 7. Improving Capacitive Load Drive
Copyright © 2011, Texas Instruments Incorporated
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